Seismologists have developed a new model of the Mendocino Triple Junction, a complex region off the Northern California coast where three tectonic plates meet. The findings, published January 15 in Science, were produced by researchers from the U.S. Geological Survey (USGS), University of California, Davis, and University of Colorado Boulder.
Amanda Thomas, professor of earth and planetary sciences at UC Davis and co-author of the study, said: “If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard.”
The Mendocino Triple Junction is where the Pacific plate moves northwest against the North American plate—forming the San Andreas fault—while to the north, the Gorda (or Juan de Fuca) plate subducts beneath North America. Previous understanding was challenged when a magnitude 7.2 earthquake in 1992 occurred at an unexpectedly shallow depth.
David Shelly, first author from USGS Geologic Hazards Center in Golden, Colorado, described their approach: “You can see a bit at the surface, but you have to figure out what is the configuration underneath.”
Shelly and his colleagues used a network of seismometers across the Pacific Northwest to detect very small “low-frequency” earthquakes. These minor events are thousands of times weaker than those felt at Earth’s surface but provide valuable information about how tectonic plates interact below ground.
The research team confirmed their model by analyzing how tectonic plates respond to tidal forces exerted by the Sun and Moon. According to Thomas: “When tidal forces align with the direction in which a plate wants to move, you should see more small earthquakes.”
Their findings suggest that instead of just three major plates meeting at this junction, there are five moving pieces involved—two of which cannot be seen from above ground. At one end of Cascadia’s subduction zone, a piece has broken off from North America and is being pulled down with Gorda as it subducts under North America. Southward, another block called the Pioneer fragment—originally part of an ancient Farallon plate—is being dragged under North America by movement along the Pacific plate.
Kathryn Materna from CU Boulder explained that this revised model clarifies why certain earthquakes occur at shallower depths than previously believed: “It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that,” Materna said. “The plate boundary seems not to be where we thought it was.”
The study received support from a National Science Foundation grant.
